This game is something BTS would honestly develop. They're so into their music and sharing it with their fans that this games gives you the chance to further appreciate their music.
There is no easy or hard song in this game because as you go through the modes you truly see how many beats of every instrument has, the flow of rhythms and ripples of their voices and tunes, and literally how many syllables Suga can rap in 2secs. You can't really complain about a song being too hard that you can't get perfect because the song itself is so complex.
Furthermore, you can play this game without dropping any cash. Sure the game has energy that you'll have to wait to reset and collectible cards for boosts but they're not difficult to receive as you play plus there is no pressure in the game to spend real money!
Overall, great game. You'll definitely put in a lot of effort into the game, haha